Acrylic acid is currently made commercially by the two-step oxidation of propylene. More recent, but not-yet-commercial, technology exists for manufacture of acrylic acid by oxidation of propane. Propylene is a petroleum derivative, and its price reflects the growing scarcity and rising price of oil. Propane, derived from oil or natural gas liquids, makes a convenient fuel and is generally cheaper than propylene, however, its price has risen as its use as a substitute for petroleum fuels in energy production has increased. Both propylene and propane are non-renewable resources.
It is desirable to find a renewable feedstock material, such as biomass, from which acrylic acid may be economically manufactured on a commercial scale. Although no direct route from biomass to acrylic acid exists, 3-hydroxypropionic acid (i.e., beta-hydroxypropionic acid or 3HP), which is a hydroxycarboxylic acid and is prepared by fermentation of sugar(s), has the potential to be such a renewable feedstock material. Although lactic acid (2-hydroxypropionic acid) is a natural product and more readily available, it is difficult to dehydrate it to acrylic acid in good yield because of competing decarbonylation and decarboxylation reactions which occur simultaneously with dehydration. On the other hand, the dehydration of 3-hydroxycarboxylic acids is accomplished relatively easily. 3-hydroxypropionic acid is not yet commercially available, but bioengineers are working to develop bacteria and yeast that are capable of converting carbohydrates to 3HP in consistent high yields. Due to the sensitivity of such bacteria and yeast to pH levels, such fermentation processes are likely to produce a salt of 3HP, for example, the ammonium salt of 3HP, in aqueous solution.
U.S. Pat. No. 2,469,701 (Redmon) teaches a method for the preparation of acrylic acid (AA) by heating 3-hydroxypropionic acid (3HP) to a temperature between 130° C. and 190° C., in the presence of a dehydration catalyst, such as sulfuric acid or phosphoric acid, under reduced pressure. The best yields of AA from this process are reported to be about 86.4%, based on assumed 100% 3HP, and 77.4% based on ethylene cyanohydrin. However, no mention is made of converting salts of 3HP to AA, and, therefore, nor is there any discussion or guidance concerning recovery and recycle of the salt-forming neutralizing agent to form additional quantities of the salt of 3HP.
U.S. Patent Application Publication No. 2005/0222458 A1 (Craciun, et. al.) provides a process for the preparation of AA by heating 3HP or its derivatives obtained from microbial or plant cells. Examples are provided which demonstrate vapor-phase dehydration of 3HP in the presence of dehydration catalysts, such as packed beds of silica, alumina, or titania, and provide high yields (80.6% to 98.2%) of AA. No disclosure or suggestion, however, is made in this published application of using salts of 3HP as the initial feed material, nor how one would recapture, and reuse a salt-forming neutralizing agent to form more of the salt of 3HP.
U.S. Patent Application Publication No. 2005/0221457 A1 (Tsobanakis, et. al.) discloses a process for preparing a salt (or ester) of acrylic acid by heating the salt (or ester) of 3HP. However, the examples of this process provided concern only the conversion of sodium and calcium salts of 3HP at elevated pressure, and they achieve poor yields (41.8 and 48.7%) of AA. Furthermore, no description or guidance is provided concerning separation and recovery of a salt capable of forming the salt of 3HP used as the initial feed material.
U.S. Pat. No. 7,186,856 (Meng, et. al.) teaches a process for producing AA from the ammonium salt of 3HP, which involves a first step of heating the ammonium salt of 3HP in the presence of an organic amine or solvent that is immiscible with water, to form a two-phase solution and split the 3HP salt into its respective ionic constituents under conditions which transfer 3HP from the aqueous phase to the organic phase of the solution, leaving ammonia and ammonium cations in the aqueous phase. The organic phase is then back-extracted to separate the 3HP, followed by a second step of heating the 3HP-containing solution in the presence of a dehydration catalyst to produce AA. Thus, the process described in this published application cracks and frees the 3HP from the aqueous phase, permitting extraction of the 3HP into the organic phase, while leaving the ammonia behind in the aqueous phase. Although the examples provided in this published application allege AA yields from 32 to 90%, applicants were unable to reproduce these results when the 3HP solution was prepared from splitting the ammonium salt of 3HP, instead of using free 3HP. Furthermore, even if the claimed yields were obtainable, this process is undesirable because the salt-splitting step adds cost to the overall process by requiring additional materials, unit operations, and equipment, as well as by generating additional waste.
In view of the foregoing existing technologies, a process for efficiently converting ammonium salts of hydroxycarboxylic acids to unsaturated carboxylic acids, such as AA, on a commercial scale is still needed. The objective of the invention is to provide an economical process for converting the ammonium salts of hydroxycarboxylic acids to corresponding unsaturated carboxylic acids, such as AA A further objective is to provide a process which includes a method for recycling the ammonia derived from acidifying the ammonium salt of hydroxycarboxylic acid.